The Khmer Rouge's chief executioner, who ran a notorious torture centre during Cambodia's genocide of the 1970s, has said he will testify in court against his former commanders.

A lawyer for Kang Kek Ieu, otherwise known as Duch, told Reuters news agency that his client would name the Khmer Rouge leaders who ordered the execution of an estimated 16,000 people at the S-21 prison in Phnom Penh.

He wants to tell the courts and the people about the truth of the Khmer Rouge regime

Lawyer for Duch

The move follows the decision by Cambodia's Constitutional Council to back the setting up of a special tribunal to prosecute members of the Khmer Rouge, who were responsible for the deaths of almost two million people in the mid-1970s.

It is unclear when the trials - to be held under joint national and United Nations auspices - will begin, but Prime Minister Hun Sen says he would like to see prosecutions by the end of the year.

UN officials have given a cautious welcome to the development, although they have warned they will back out of the whole process if it excludes key figures in the Khmer Rouge regime.

Duch and a Khmer Rouge military commander Ta Mok, who were arrested in 1999, are the only members of the Khmer Rouge in detention awaiting trial.

"He wants to talk. He wants to tell the courts and the people about the truth of the Khmer Rouge regime," he said, adding that Duch had been cheated into the killings by the leaders Pol Pot, Nuon Chea and Khieu Samphan.

The lawyer said that if Duch had refused to be the chief of the S-21 prison, the Khmer Rouge leadership would have killed him.

"When Duch joined the Khmer Rouge his aim was not to kill people, only to make a clean and just society," he added.

S-21, once a Phnom Penh high school, is now a genocide museum.

Whitewash fears

After a two-hour meeting on Monday, Cambodia's Constitutional Court cleared the way to create a court to try Khmer Rouge members.

The legislation now needs to be signed by King Norodom Sihanouk to become law, and must also be approved by the UN.

Khmer Rouge leaders

Pol Pot: Died in 1998

Ta Mok: The Butcher, captured and awaiting trial

Kang Kek: Chief executioner, in jail awaiting trial

Ieng Sary: Foreign minister, pardoned

Nuon Chea: Chief political theorist and "Brother Number Two", at liberty

Khieu Samphan: Public apologist, at liberty

The special courts will be made up of three Cambodian and two foreign judges.

But there are some who believe the trials will be a whitewash, because many of the most notorious Khmer Rouge leaders have already been given amnesty under a deal in the 1990s to end the country's long-running civil war.

The Khmer Rouge leader, Pol Pot, died in 1998.

Since the mid-1990s there has been much talk about punishing those responsible for the so-called Killing Fields, where 1.7 million people died at the hands of the Khmer Rouge between 1975 and 1978.

The BBC's Jonathan Head in Phnom Penh says there may be concerns about such trials among members of the current Cambodian Government, some of whom formerly belonged to the Khmer Rouge.

Many Cambodians have yet to put the past behind them

Four years ago, Cambodia asked the United Nations for help in establishing a special tribunal to judge the architects of genocide, but agreement on how it should be set up and run has been elusive.

The UN wanted a panel of international judges, sitting outside Cambodia, to run the tribunal; the Cambodians wanted only local judges on the panel.

In the end, a compromise was reached.

Under the agreement, trials will be held on Cambodian soil, but the UN is insisting that international standards of justice must be met when trials begin.

But until they get to scrutinise the legislation, UN officials are not willing to give the court their stamp of approval.

BBC UN correspondent Greg Barrow says that without the world body's support, a tribunal charged with trying those responsible for some of the greatest crimes against humanity will lack international credibility.

Divisive issue

The issue of whether or not there should be prosecutions is a divisive one for Cambodia.

While some want all the top Khmer Rouge leaders who are still alive in the dock, there are others being more cautious.

Hun Sen told the BBC that if handled incorrectly, the trials might re-ignite civil war, especially if Khmer Rouge leaders who gave themselves up under the amnesty deal are prosecuted.

But in the eyes of his critics, the prime minister is simply trying to keep out of jail those who can help him politically.